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COLEY-SMITH,. R. (1960). Ann appl .
Bbl
48
(I), 8-18.
STUDIES
OF
T H E
BIOLOGY
OF
SCLEROTIUM
C E P I V O R U M
BERK.
IV.
G ERMIN A TIO N
OF SCLEROTIA
BY J.
R.
COLEY-SMITH
Department
of
Botany, Universi ty
of
Birmingham
(With Plate
I
and z Text-figures)
Sclerotia from 6-week-old pure cultures of Sclerotium cepivorum germinated
immediately in soil only after abrasion of their rinds, but after burial in soil for a
month or more, unabraded sclerotia became capable of germination.
Marked stimulation of germination occurred in the presence of host plants
(onion, leek and shallot). Little or no germination occurred in soil alone or in the
presence of non-host plants (barley, cabbage, carrot and white clover).
Sclerotial
germination was observed in a number of soils of widely differing pH and over
a wide range of soil water content. Germination of sclerotia on uninjured onion
roots was greatest at the tip region. On artificially injured roots sclerotial germi-
nation was enhanced but the effect of position was lost.
Sclerotial germination was independent of contact between roots and sclerotia.
It was induced by root extracts of all
Allium
spp. tested, but of no other plants.
Boiling or autoclaving root extracts did not destroy the active principle and
it is concluded that under field conditions sclerotia are induced to germinate by
a thermostable chemical substance from All ium roots.
The process of germination of sclerotia is described.
INTRODUCTION
In a previous paper (Coley-Smith, 1959) sclerotia of Sclerotium cepivorurn Berk.
were shown to persist in soil under field conditions, for periods of up to 4 years.
However, in order to function satisfactorily as perennating bodies sclerotia must
not only be capable of persistence in the absence of host plants, but must also be
able to germinate and infect the host, under appropriate circumstances. Scott (1954)
discovered that sclerotia from 6-week-old pure cultures would germinate in soil
only after artificial abrasion
of
their rinds. Coley-Smith Hickman (1957)
reported that the presence of onions in soil stimulated germination of suitably
aged sclerotia and suggested that the effect was a chemical one. The present paper
describes further experiments on the influence of the host and its extracts on
sclerotial germination.
MATERIALS
N D METHODS
The isolate of Sclerotium cepivorum and method of production of sclerotia were those
used previously (Coley-Smith, 1959). Unless stated otherwise, sclerotia were
buried for
6
months in
soil
in a glasshouse before use. In some experiments
*
Now
at
Department of Hop Research,Wye College (University of London), near Ashford, Kent.
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Biology
of Sclerotium cepivorum Be . IV
9
sderotial germination was measured indirectly, as before, by counting infected
seedlings but in the majority of experiments direct observations were made of
germination on a
10
x
2.5
em. strip of nylon material (average mesh size
128p
placed inside a
4
x
19
in. glass specimen tube, near the top, and moistened
so
that
the nylon stuck to the glass.
Sclerotia were then placed with forceps in a circle
between the nylon and the side of the tube. To examine the effect of seedling onions
(var. White Lisbon) and other plants on sclerotial germination, seeds were placed
in another circle
I
cm. above the sclerotia.
After addition of soil both sclerotia
and seeds remained in position and the tubes were incubated at a constant water
content (Keen Raczkowski, 1921 . Black paper secured around the outside of
the tubes discouraged the growth of algae.
In the field, a glass tube 12
x
I in. was used, permitting root growth over a long
period. The bottom of the tube was plugged with a nylon-covered cork, pierced
with a
I
cm. diameter hole which was plugged with soil. This allowed free drainage
yet prevented growth of roots into the surrounding soil and hence their injury
during removal of the tube for examination. Sclerotia were placed on a strip of
nylon near the top of the tube
and
onion seeds grouped similarly I cm. higher up.
The tubes were then filled with soil and buried up to their rims under natural
conditions in the field.
EXPERIMENTAL
ESULTS
In the early part of this work Scott's (1954)statement, that sclerotia from 6-week-
old cultures would germinate in soil only after their rinds had been artificially
damaged by abrasion, was confirmed on numerous occasions. Sclerotia which
were ground gently with a pestle and mortar until their rinds were broken germi-
nated readily on nylon strips in soil tubes, whereas undamaged ones did not. On
agar media sclerotia of all ages germinated equally readily, without injury.
In tests using single, abraded sclerotia low levels of infection always resulted
but infection was raised by increasing the number of abraded sclerotia per onion
seedling. However, when unabraded sclerotia which had been buried in the field
for
24
years were used singly as inocula very high levels of infection occurred. T o
find how long a period in soil was necessary before unabraded sclerotia became
infective, the following experiment was made.
After inoculation with 6-week-old sclerotia, which had been buried in pots of
soil under glasshouse conditions for periods of from I to 6 months, White Lisbon
seedlings were placed in the laboratory (6070 F.) in tubes of Edgbaston garden
loam (pH 7.0) at 40 water-holding capacity (hereafter referred to as w.h.c.).
Records of infection are shown in Table I . No further infection occurred after
8 weeks.
Sclerotial dormancy started to break down shortly after burial in soil and after
2
months it had almost,
if
not completely, disappeared. This result has been
confirmed with sclerotia buried under field conditions.
In the above experiment, and in others, high levels of germination (usually
6-2
)
were obtained by the use of the infection technique with suitably aged
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I 0 J. R.
C O L E Y - S M I T H
sclerotia. On the other hand germination of sclerotia on nylon strips in soil tubes,
without onions present, was usually poor 0-15
yo).
This suggested a stimulatory
effect of the host on sclerotial germination and the following three nylon strip
experiments were designed to examine this possibility. In each of these experi-
ments there were three tubes per treatment, each with twelve sclerotia and twelve
seeds. Replicate values have been summed for convenience of presentation.
No
germination
of
sclerotia occurred in control tubes without seeds and these have
been omitted from the tables of results.
TABLE
.
Infection
of
W hi te Lisbon onion seedlings
by Sclerotium cepivorum
sclerotia buried beforehand
f o r carious
periods in soil
Duration of
previous burial
of sclerotia
in months
OX
I
2
3
4
5
6
h'o. of seedlings infected
Weeks
I 2
3 4
5 6
0
0 0
I 2
2
3
0 I 2 3 9 I 1
0 3
8 9
1 0
I3
0 I
5 7 8 I 1
0 I 6 10
I 1 I 2
0 I 1 I 2 I 2
3
1 0
*
Direct from culture.
7 8
3
5
13
I 3
I 1
I1
I 1
I 2
I 2
I 2
I 2
I 2
T A B L E
2 .
Germination of 6-month-old sclerotia of
Sclerotium cepivorum
in
Edgbaston garden loam in the presence
of
onions, leeks and shallots
No. of
? water-
No.
of No. f sclerotia germinated Final yo seedlings
holding seeds Weeks sclerotial infected
Plant of soil nated I
2 3
4
5 6 7 8
nation white rot index
L
capacity germi- ------ i germi- by Infection
Onion 40 26 0 0 0 0 1 4 6 6 16.7 23 1.44
60
26
o
o
o
I
4
9 10
1
27.8
60
25
o I
I 2
3
6
8
10 27.8
Leek
40 26
0 0 1 4 5 7 9 9
2 j . o
3 0.16
Sha l lo t
40 2 j
0 0 0 5 6 7 9 9 25.0
8 0 3 3
60
25
I 5 9
I 0
I 3 15 15 41 7
In the first of these experiments sclerotia in tubes of Edgbaston garden loam
(pH 6.5 were maintained at 40 or 60 w.h.c. with distilled water. Seeds of the
following plants were used: onion var. White Lisbon, leek var. Musselburgh,
shallot var. Giant Red, barley var. Atlas 46, cabbage var. Harbinger, carrot var.
Early Horn, white clover var. S
roo.
No germination of sclerotia occurred in the
presence of the last four plants and these are excluded from the results shown in
Table
2.
Although there was a marked effect of the presence of
Allium
seedlings on sclerotial
germination there was little difference in the stimulatory capacities of onions, leeks,
and shallots. An examination of the infection index (number of seedlings infected
with white rot divided by number of sclerotia germinated) shows that stimulatory
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Biology
of Sclerotium
cepivorum Bwk. IV I 1
capacity and susceptibility to white rot are not necessarily related. Although onions
caused slightly less sclerotial germination than leeks the infection index for onions
was nine times as great as that for leeks. Mycelium from germinating sclerotia
persisted for about a fortnight and then disappeared, leaving the sclerotial rinds as
broken, hollow shells which themselves ultimately decayed.
Stimulation of sclerotial germination by onion seedlings was also examined in
soils of widely differing pH:
( I ) Sutton Park podsol, average pH 3-5.
(2) Kings Norton loam. A heavy clay loam; average pH 48.
(3) Edgbaston garden loam. A lighter, more sandy loam; average pH 6.0.
(4)Edgbaston garden loam to which lime (CaO) was added. The pH at the
beginning of the experiment was 8.5, dropping to
7 - 2
by the end.
In the podsol soil, seed germination and subsequent seedling growth was very
poor and there was a vigorous development of
Trichoderma viride
on both seeds
and nylon strips. After a month all the seedlings had died and no sclerotia germi-
nated. Results for this series are therefore excluded from the results in Table 3.
TABLE
.
Germination of 6-month -old sclerotia
of
Sclerotium cepivorum
in the presence of W hi te Lisbon onions in var ious soils
water-
holding
Soil capacity
Kings Norton
loam
40
60
Edgbaston garden 40
loam 60
Edgbaston garden
40
loam
plus lime 60
No. of
onion
seeds
germinated
25
3 2
23
29
21
29
I
0
0
0
0
No. of sclerotia germinated
Weeks
2 3 4 5 6 7
0 1 2 3 7 7
0 1 2 5 5 5
o
o 2 5 1 0 1 7
2
4
5 8 9 1 7
o 9 22 28
31
35
0 7
23
28 30 33
7
8
9
7
25
35
3 4
22
Final yo
sclerotial
germination
25.0
194
61.1
6 9 4
972
944
There were very large differences in the levels
of
sclerotial germination in the
three soils, the greatest germination occurring at the highest pH. This relationship
has been confirmed with a natural, alkaline soil, Lias clay (average pH 7 . 6 ) n which
germination was greater than in Edgbaston garden loam or Kings Norton loam.
To study the effect of soil moisture
on
sclerotial germination, tubes containing
Edgbaston garden loam (pH7.0) djusted to 20-100 w.h.c. were set up. Germi-
nation of onion seed was satisfactory over the whole range except at the 100
level, where it was very poor. Results for this water level are consequently not
included in the results (Table
4).
There was a general rise in sclerotial germination with increasing soil moisture
content. On the other hand the pattern of onion seedling infection was very
different, the infection index being highest at moderate water levels with a tendency
to fall
off
at both extremes
of
the range.
Since the presence of the host had such a marked stimulatory effect on sclerotial
germination in the laboratory, attempts were made to discover whether similar
effects occurred in the field.
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I 2 J. R.
C O L E Y - S M I T H
Eighteen glass germination tubes as described earlier, each with ten sclerotia
and filled with Edgbaston garden loam (pH 6.9), were set up. In six of these
eight White Lisbon seeds were grouped in the usual manner. I n another
six
tubes
a single seedling at the second true-leaf stage was planted in the centre of each,
and the remaining six were left without onion seeds as controls. The tubes were
buried up to their rims in the field on I April 1958. Apart from periodic examinations
they were left under completely natural conditions. A record of sclerotial germi-
nation is shown in Table
5 .
Tubes without onions are not included since no germi-
nation occurred in them. In the other two treatments replicate values have been
summed.
TABLE
.
Germination of 6-month-old sclerotia
of
Sclerotium cepivorum
in
the
presence
of
White Lisbon onions, in Egbaston garden loam adjusted to
& feerent
moisture levels
No, of
No. f No. of sclerotia germinated seedlings
O 0 water- onion Weeks Final o infected
capacity germinated I 2 3 4 5 6 7 8 germination white rot index
h
holding seeds sclerotial by Infection
20
30
40
5 0
60
70
80
90
32 0 0 0 1 2 3 3 3 8.3
31 0 0 0 0 2 ~ ~ 4
1'1
33 0 0 1 3 3 j 6 6 16.7
33 0 0 0 0 1 6 8 8
22 2
28 o o o 5 7
1 0
1 1 1 3 36.1
33 0 0 3
7 1 1 1 1 30.6
3 1
o o 2 7
I I
16 17 17 47 3
26
o
o
3
16
19
26 27 27 75 1
I
3
12
5
15
I 2
22
2
0 3 3
0 75
1.88
0 92
1.36
1.29
0.74
2 00
TABLE. Geminat ion
of
6-month-old sclerotia of Sclerotium cepivorum in tubes
of
Edgbaston garden loam, in the field,
in
the presence
of
W hit e Lisbon onions
No.
of
sclerotia germinated
A
I
7
Final
18 28 1 3 ZI 27 2 9 16 2 3 3 7
21
I I sclerotial
Treatment
.4pr. Apr. May May May June June June June July July July Aug. germination
73 3
onion seedlings o o
2
1 3 28 37
f
--
per tube
per tube
I onion seedling o o
o
I I I
2 j 6 7
13 14
28 46.7
* Tubes abandoned, all onion seedlings dead.
In the series with eight onions per tube 56.7 yoof the seedlings became infected
with white rot, and in the treatment with a single onion per tube two of the six
seedlings were infected. This experiment proved that stimulation of sclerotial
germination followed by infection of onion seedlings can occur under field conditions.
A
criticism which could be applied to the nylon-strip method for studying sclerotial
germination is that
it
does not simulate fully the conditions which sclerotia would
encounter in nature completely surrounded by soil. An experiment was therefore
devised to discover whether onions have any effect on sclerotia dispersed naturally
in soil.
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Biology
of
Sclerotium cepivorum
Berk.
IV
13
On e hundred sclerotia were mixed in each of a num ber of 4 4 in. glass speci-
men tubes containing Edgbaston garden loam pH 6.9). Two types
of
sclerotia
were used:
I )
From
a
6-week-old sand corn-meal culture.
(2) From 6 months burial in soil.
T en onion seeds were sown in half of th e tubes and w ere replaced with fresh ones
when seed lings infected with white rot were observed. All tubes were kept und er
laboratory conditions at 40 w.h.c. Sclerotia were recovered by the sieving
method described previously Coley-Sm ith, 1959). T h e final recovery figures
after
I
year are shown in Table 6.
TABLE. Persistence of sclerotia
of
Sclerotium cepivorum in Edgbuston garden
loam
in the presence or absence of W hi te Lisbon onions
With + ) or Average
without
(-)
No.
of
sclerotia recovered in three recovery
of sclerotia
onions
replicates of sclerotia
6 weeks + 5 1 68 27 48.7
I 0 0 95 96 97 0
94 98 88 93 3
-
6
months
+ 72 8
17
32 3
-
Despite th e large variation between replicates there was
a
marked effect of the
host on sclerotial numbers and this was obviously due to stimulated germination.
Frequent infection
of
onion seedlings occurred and in a few instances sclerotia
were observed in a germ inating condition during recovery.
I n th e foregoing experiments it was often noted tha t sclerotial germination was
enhanced in th e vicinity of decaying seedlings, which suggested tha t host injury
might be important. An experimen t was designed to exam ine this possibility.
W hite Lisbon seeds were grown for 5 days at 20
C.
in Petri dishes lined with
mo ist filter-paper. Sclerotia from 6 -months soil burial were placed on the roots
of the seedlings in th e following positions:
I )
On the root tip.
(2) Midway between the root tip and the developing bulb region.
( 3 )
On
the developing bulb region.
In half of the dishes seedlings were uninjured a nd in the remainder the root surfaces
were punctu red w ith a needle, sclerotia then being placed on the w ounded area.
Th ere were forty sclerotia per treatment, twenty in each of duplicate dishes. Th ey
were transferred to fresh seedlings every 5 days and exam ined microscopically for
germination. Results are shown in Tab le
7,
duplicate values being summed.
No germination occurred of control sclerotia placed on moist filter-paper.
O n un injured roots there was a very marked positional effect, sclerotial germina-
tion being greatest at the root-tip region. Root injury whilst bringing about an
increase in germination destroyed the positional effect.
Another phenomenon occurring in the soil experiments described was the
freque nt germination of sclerotia at distances
of
up to a centimetre from the
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I 4 J.
R.
C O L E Y - S M I T H
nearest host root. This suggested a chemical stimulus rather than a contact effect.
Experiments were designed to investigate this point more fully and to provide
further information on the nature of the stimulus involved.
Sclerotia were placed on moist filter-paper in Petri dishes. In half of the dishes
they were positioned inside glass rings
I
in. diameter) resting on the surface in the
centre of the dishes, and in the remainder on filter-paper in subsidiary dishes
enclosed within the Petri dishes. White Lisbon onion seeds 1-5
.)
were scattered
around the outer portion of half of the dishes, contact with sclerotia being prevented
TABLE
.
Germination q 6-montlz-old sclerotia
of
Sclerotium cepivorum
a t 20 C . on roots of W hi te Lisbon onion seedlings
No. of sclerotia germinated
Injured or Weeks
uninjured Position
of
roots
sclerotia on
root
I 2 3 4 5 6 7 8
Uninjured
Root tip 5
21
29 38 39 39 39
39
Between tip and
3 3 16 16
17 18
19 19
On developing o 6 15 18 19 20
ZI 23
bulb
bulb region
Injured
Root
tip
19 37 40
Between tip and 1 9 3 9 4 0 - - - - -
On developing 2 3 39
40
--
ulb
bulb region
Final yo
sclerotial
germination
97 5
47 5
57 5
1
I 0 0
I 0 0
TABLE.
Germination
of
6-month-old sclerotia of
Sclerotium cepivorum
at
20
C.
on
moist
j i l ter-papm
in
the presence or absence of Wh ite Lisbon
onion
seedlings
Sclerotia on filter-paper
within glass ring in Petri
dishes (A) or in subsidiary
dish within Petri dish (B)
Total number of sclerotia
germinated (out of 240)
after
14
days incubation
With (+ ) or
without ( - )
onions
+
A
R
A
B
7
I 2
2
I
either by the glass rings or the subsidiary dishes. In the former case stimulation
could occur by diffusion of the stimulant through the filter-paper, but the sclerotia
in subsidiary dishes could only be stimulated if the active principle were volatile.
Dishes were incubated at 20
C.
and were kept moist with distilled water. Non-
viable seeds were removed daily to prevent excessive contamination of the dishes by
saprophytic moulds. Three experiments of this type were carried out, in each of
which there were four dishes per treatment with twenty sclerotia per dish. The
experiments were performed under identical conditions and the final germination
figures, recorded after 14
ays incubation, have been summed in Table
8.
The results show that contact between sclerotia and host roots is not necessary
for stimulation of sclerotial germination. The active principle was apparently able to
pass quite readily through moist filter-paper but was only slightly
if
at all volatile.
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Biology
of
Sclerotium cepivorum Berk.
IV
15
Since contact with host roots
is
not necessary for stimulation of sclerotial
germination it should occur in the presence of root extracts or exudates from living
roots. This possibility
was
examined by treating sclerotia with root extracts of
various plants.
Root extracts were made from 6-week-old seedlings by grinding 10 . fresh root
tissue to pulp, suspending in 250 ml. distilled water, and filtering
off
the debris.
The following plants were used
:
onion var. White Lisbon, leek var. Musselburgh,
shallot var. Giant Red, cabbage var. Harbinger, and brussels sprout var. Cambridge
no.
5.
Nylon-strip germination tubes were set up, filled with Edgbaston garden
loam (pH
7.0)
and adjusted to 40yow.h.c. with distilled water. They were incubated
under laboratory conditions and the original water levels maintained by the
addition every
2
days of the appropriate root extract (approximately
2
ml.). There
were three tubes per treatment, each with ten sclerotia. No germination of sclerotia
occurred in the distilled-water controls or in those tubes which received brussels
sprout or cabbage extracts. It was entirely confined to the tubes which were
watered with Allium extracts, where germination figures were consistently high.
Summed values
of
replicates are shown in Table
9.
TABLE
. Germination of 6-month-old sclerotia of Sclerotium cepivorum in
Edgbaston garden loam watered wit h root extracts of onion, leek
and
shallot
No. of sclerotia germinated
Weeks Final
?A
-
L
> sclerotial
Extract
I 2
3
4
5 6
7 8 germination
Onion
0 0
3
8
I
20
23
26 86.7
Leek 0 0
2 6
8
I4
20 22 73'3
Shallot
0
4 I 1
I 3
2 1
26 28 93 3
TABLE0. Germination of 6-month-old sclerotia of Sclerotium cepivorum in
Edgbaston garden loam watered with
root
extracts of Allium spp.
No.
of sclerotia germinated
Weeks Final
A
r clerotial
Extract
1
2
3
4 5 6
7
8
germination
Onion
0 0
0
0 I
5
7 7 23'3
Crow garlic
o
0
9 1 1 12 16
22 22 73'3
Garlic
0 0
5
7
20
23
24 24
80.0
Ramsons
o 0 I 2
I 0
24
26 26 86.7
In another experiment, set up in the same way, root extracts of the following
plants were used: beetroot var. Crimson Globe, carrot var. Early
Horn,
barley var.
Atlas 46, onion var. White Lisbon, crow garlic (Alliu m vineale), garlic (unknown
variety), ramsons ( A . ursinum), and garlic mustard (Alliar ia petioluta). Th e first
four of these were 6-week-old seedlings, and the remainder were plants of unknown
age from the field. Again germination of sclerotia was confined to those tubes
watered with extracts of Allium spp. (Table
10),
with the exception of a single
sclerotium in a tube which received garlic mustard extract. No explanation can
be given for the low levels of germination with onion extract.
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16
J. R. C O L E Y - S M I T H
In the final experiment to be described the effect of boiling and autoclaving root
extracts was investigated.
The extract was made from roots of I-month-old White Lisbon seedlings by
grinding 20
g.
fresh tissue to pulp, extracting with distilled water and making up
to 500ml. The extract was divided into five IOO ml. portions which were treated
as follows:
( I )
Untreated.
( 2 ) Seitz-filtered.
(3) Autoclaved for 15 min. at 15 lb./in.2 pressure.
4) Boiled for 5 min.
5)
Boiled for
15
min.
TABLE
I .
Germination of 6-month-old sclerotia of Sclerotium cepivorum in
Edgbaston garden loam
(pH
7 2 )
watered with W hi te Lisbon root ex tract wbjected
to various treatments
Treatment I
Untreated extract 0
Seitz-filtered extract
Autoclaved extract 0
Extract boiled
5
min. 0
Extract boiled 15 min. 0
No.
of
sclerotia germinated
Weeks
2
3
4
I
I 3 23
0 r r 14
I
17 20
3
15 23
I
14
I S
Final
sclerotial
germination
76.7
46.7
66.7
60.0
76.7
After treatment volumes were restored with distilled water. The extracts were
added to sclerotial germination tubes every
2
days to maintain these at
40
w.h.c.
There were three tubes per treatment, each with ten sclerotia. Records of sclerotial
germination are shown in Table I I , replicate values having been summed.
None of these treatments appeared to have had any very marked effect on the
potency of the extract and it is presumed that the active principle is thermostable.
PROCESS
F
GERMINATION
As
the process of germination of sclerotia of Sclerotium cepivorum has not before
been described, sclerotia for anatomical study were obtained from the foregoing
germination experiments. They were either mounted in water for microscopical
examination or fixed, dehydrated, embedded and sectioned.
Mature sclerotia
of
S. cepivorum possessed an outer rind of one to two layers of
much-thickened, heavily pigmented rounded cells, enclosing a large medullary
region consisting of fairly closely packed elongated hyphae.
The first external sign of germination was the appearance of a bulge on the
sclerotial surface. After this the rind was ruptured, presumably by pressure from
within, and a large dense plug of mycelium was extruded (Fig. I A, 3, C).
In
some
of the root-extract germination experiments described previously, especially in the
presence
of
Allium sativum,
A .
vineale, and A ursinum extracts, as many as three
large plugs were formed by a single sclerotium. The hyphal plug originated from
the medullary region, apparently by elongation of already existing cells.
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Biology
of Sclerotium cepivorum
Berk.
IV
7
Fig.
I.
Stages in germination of
S.
cepivorum sclerotia, from nylon strips in soil tubes,
showing rupture of the rind A followed by extrusion of a plug of mycelium
B,
C, D.
-2
Fig.
2.
Stages in germination of
S. cepivorum
sclerotia, from nylon strips in soil tubes.
A development of microconidia on exposed medullary region of sclerotium; B, early
stages in sclerotial initial formation ;C,
D,
later stages in formation of sclerotial initials ;
E, sclerotial initials and sporodochia with microconidia and a tuft of microconidia on
a
short
side branch;
F,
microconidia, borne in verticils;
G,
phialides with microconidia
borne direct upon a hypha.
2
App. Biol.
48
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I 8 J. R.COLEY-SMITH
Shortly after emergence of the sclerotial plug, hyphae grew out from the latter
and anastomosed freely (Fig. I D). Ultimately the hyphae grew to distances
of
up
to several millimetres from the sclerotial body. Frequently the latter was torn apart
by the force exerted by the emerging hyphae, thus exposing the central medullary
region, which often became covered with tufts of microconidia (Fig. 2A).
Two
or three days after commencement of germination, sclerotial initials were
produced at the tips of many hyphae (Fig.
zB , C,
D, E) as described from agar
cultures by Townsend Wiiletts (1954).Only on rare occasions were sclerotial
initials ever observed to form mature sclerotia, although coalescence of small
initials to form larger ones was quite common.
In addition to sclerotial initials microconidia were also produced on the germi-
nation hyphae. These were very small (16-z-op in diameter) and were borne in
chains on flask-shaped phialides which occurred in verticils [Fig. zF), on short
side branches (Fig.
2E) ,
or in tufts directly upon a hypha. Sometimes an actual
cushion or sporodochium was formed on which microconidia were borne in vast
numbers (Fig. zE). Microconidia were never observed to germinate and their
function, if any, is unknown.
Dissolution of the hyphae and sclerotial initials occurred some 10-12 ays after
germination and ultimately the sclerotia were left as broken hollow shells, the
central medullary region having disappeared entirely.
This investigation
was
carried out during the tenure of
a
grant from the Depart-
ment
of
Scientific and Industrial Research.
I
wish to thank Dr
C.
J. Hickman for
his invaluable advice and encouragement.
REFERENCES
COLEY-SMITH,
R .
(1959).
Studies of the biology of ScZerotium cepivorum Berk.
111.
Host
COLEY-SMITH,.
R. HICKMAN,
. J.
(1957).
Stimulation of sclerotium germination in
KEEN,B.
A. RACZKOWSKI,. (1921).
The relation between the clay content and certain
SCOTT,
M.
R.
(1954).
The
biology of Schot ium cepivorum
Berk. Thesis for Ph.D., Uni-
TOWNSEND,RENDA. WILLETTS,. J.
(1954).
The development of sclerotia of certain
range; persistence and viability
of
sclerotia.
Ann . appl . Biol
47,
I
I .
Sclerotium cepivorum
Berk.
Nature,
Lond., 180,
445.
physical properties of
a
soil. J . a . Sci 11,441,
versity
of
Birmingham.
fungi.
Trans. Brit.
mycot. SOC. 7, 213.
EXPLANATION OF P L A T E 1
A sclerotium of Sclerotium cepivorum Berk. germinating on a nylon
strip near an onion root.
(Received z M a y 1959)